Submarine slope failures are common in gas hydrate bearing sedimentary basins because hydrates reduce sediment permeability and cause build-up of over pressure at the base of the Gas hydrate stability zone. This overpressure results in formation of hydro fractures/faults as a pathway for over pressurized fluids to migrate upwards. When these pathways reach shallow permeable beds, this overpressure transfers laterally and destabilizes the slope. This process reconciles the spatial correlation of submarine landslides and gas hydrates. The Krishna Godavari basin in India is a passive margin basin which shows higher potential for gas hydrate accumulation. Shale reservoirs in KG basin has less permeability compared to sandstone reservoirs therefore the free gas that is locked in pore spaces of shales would eventually create hydro fractures or faults to escape from the pore spaces. There are two such faults identified in the study area at water depths approximately 1040 meters. The faults are interpreted using the post stack seismic data and the BSR (Bottom simulating reflector) which is a primary marker of Base of gas hydrate stability zone are identified in the seismic data. The depth of BSR is approximately 160 meters below sea floor which is further confirmed using well log data of NGHP-01-10A. From seismic attribute analysis, we conclude the presence of free gas below BSR and presence of faults (fault A and fault B), which act as primary migration pathways.
Fault B is developed due to overpressure and passes through the BSR and the hydrate stability zone as can be evinced from the pockmarks and Mass Transport Deposits. Fault A shows small signatures of vertical fluid flow as disruptions in the BSR, from a critically pressurized gas below the BSR. Fault A is not an active migration pathway as it is capped by young sediments and can become potential pathway if the gas column thickness below the Fault A exceeds 51m. Fault B is an active migration pathway because the series of pockmarks are present above the fault which indicates the ongoing advection in the study area. When these pockmarks are interconnected it will create a slump in the area by causing slope failure. There are two existing slope breaks in the study area named as SB1 and SB2. Therefore, it has been concluded that the faults are acting as a primary migration pathway in the study area.
Citation: Palle, J., Nittala, S. & Samudrala, K. Gas hydrate/free gas migration pathways in submarine slope failures: East Indian Margin. Journal of earth system science 130, 83 (2021). https://doi.org/10.1007/s12040-021-01575-5.